PROJECT SUMMARY The development of novel cartilage regeneration and osteoarthritis (OA) treatments is limited by our incomplete understanding of how certain molecular activities in cartilage extracellular matrix (ECM) are linked to its matrix assembly and biomechanical functions. This project will study the activities of decorin, a small leucine rich proteoglycan (SLRP), which appears to be critical for the assembly and stability of the aggrecan network, a major constituent of cartilage ECM. The overall objective is to determine the roles of decorin in regulating the assembly of the aggrecan network in healthy and degenerative cartilage. Our central hypothesis is that decorin acts as a ?physical linker? to regulate the assembly of the aggrecan network in cartilage ECM, and this role enables the establishment of the normal biomechanical function of cartilage during joint development, and delays cartilage degeneration in OA by increasing the retention of fragmented aggrecan. We will elucidate the roles of decorin in normal cartilage ECM assembly (Aim 1) and in OA-associated cartilage degradation (Aim 2), respectively. In Aim 1, we will first determine whether decorin is essential for the proper biomechanical function of normal cartilage during post-natal growth. Next, we will determine whether decorin increases the retention of aggrecan in the forming neo-matrix of chondrocytes under dynamic loading, and if decorin also regulates chondrocyte anabolic response to growth factors. In Aim 2, we will first determine whether decorin slows down cartilage degradation and OA progression using two murine OA models, including naturally occurring OA in aged mice and injury-induced post-traumatic OA in the destabilization of the medial meniscus (DMM) model. Next, we will determine if decorin increases the retention of aggrecan fragments in degenerative cartilage, and if decorin also directly impacts chondrocyte catabolism in response to inflammatory factors. A number of innovative approaches will be utilized. Using cartilage-specific decorin inducible knockout mice, we will delineate decorin activities during normal cartilage homeostasis and those during the progression of OA. Applying atomic force microscopy (AFM)-nanomechanical tests, we will quantify the mechanical changes of cartilage as a result of decorin deficiency. By combining these approaches, we will elucidate the roles of decorin in regulating the structure and function of normal cartilage, and the degradation of cartilage during OA onset and progression. Successful completion of this study will establish a new structure-mechanics principle of decorin-mediated aggrecan network assembly in the cartilage ECM. This will direct our future studies to improve cartilage regeneration and to slow down OA progression by modulating the activity and availability of native decorin, as well as the design and delivery of decorin-biomimetic synthetic molecules.